1. Field of the Invention
The present invention relates generally to optical systems that utilize optical fiber for propagating high power optical signal, and more particularly to lasers, laser amplifiers and oscillators with a rare earth doped optical fiber operating in high power regime.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Single clad rare earth doped optical fiber has been widely used in the field of optical amplifiers and fiber lasers. This type of fiber has relatively low capability of handling high power multimode optical sources due to the difficulty of efficiently coupling multimode light from a high power optical (light) source (also referred to herein as optical pump or pump) into the rare-earth doped fiber core.
To solve this problem and to increase the output power of fiber lasers, those of skill in the art utilize optical fiber with a double clad structure (referred herein as double clad optical fiber). Double clad rare-earth doped optical fiber is a fiber that has a core, an inner cladding layer surrounding the core and an outer cladding layer surrounding the inner cladding layer. Optical fibers with Yb doped cores and two cladding layers surrounding the core are disclosed, for example, in U.S. Pat. Nos. 6,477,307; 6,483,973; 5,966,491 and 5,949,941.
Double clad optical fiber has been used in applications requiring utilization of optical sources providing between 10 to 100 Watts of optical power, because double clad optical fiber is more efficient in retaining/utilizing optical power provided by the optical pump than single clad optical fiber. This higher efficiency is due to fiber's utilization of clad-to-core coupling of optical pump power. More specifically, rare-earth doped double clad optical fibers accept light from the optical pump into the inner cladding and then transfer light to the rare-earth doped core through the core to inner cladding interface, along the length of the optical fiber. Thus, the optical fiber converts a significant part of the multi-mode light propagated through the inner cladding into a single-mode output at a longer wavelength, by coupling the pump light into the rare-earth doped core.
The inner cladding of the double clad optical fiber has a higher index of refraction than the outer cladding, thus the pump energy is confined inside the inner cladding and is re-directed into the core. The optical fiber is optically active due to the presence of rare-earth dopant in the core, which can be excited to higher electronic energy levels when the optical fiber is pumped by a strong optical pump. The core is typically doped with at least one rare-earth element, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium- or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030 nm-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the optical fiber using a multi-mode inner cladding for accepting and transferring pump energy to the fiber core along the length of the fiber. Cladding pumping can be utilized in fiber amplifiers, or employed to build high-power single mode fiber pump lasers.
In fiber laser applications rare-earths such as Nd, Yb or Er have three-level transitions that require at least 50% of total ions to be in the upper level (i.e., at least 50% inversion) to exhibit gain. These three level transitions result in production of gain (or lasing) in these wavelength ranges: 880-920 nm in Nd doped optical fibers, the 965-1020 nm in Yb doped optical fibers, 1510-1540 nm in Er doped optical fibers. All of these are earth ions have competing 4-level transitions that require much lower level of inversion to exhibit gain, as low as 3% to 4%. The wavelengths corresponding to the 4-level transitions are 1050-1100 nm transitions in Yb and Nd doped optical fibers, and the 1560-1600 nm transitions in Er doped optical fibers. For a given length of the optical fiber and a given double-clad geometry, four level transitions reach transparency (i.e. the condition when gain reaches the loss in the fiber) or laser threshold at a much lower pump power level than three-level transitions. Once the optical fiber exhibits gain or starts lasing as a 4-level system, it does not operate as a 3-level system, and provides the gain or lasing wavelength in an undesirable wavelength range.
The problem of unwanted 4-level lasing or gain is typically solved by adding filters such as dielectric filters to suppress gain in four-level transitions, or alternatively, by choosing a double-clad inner cladding area that “promotes” a higher pump power density to increase the local inversion. However, dichroic filters are difficult to make spectrally sharp enough so that no additional loss is added at three-level wavelengths. That is, such filters introduce power loss at the 3-level wavelengths. Furthermore, these dichroic filters add further complexity and expense to the overall system. Provision of higher pump density is typically achieved by utilizing optical fiber with the relatively small clad to core ratios. While this approach increases pump power density to promote three-level inversion it makes it difficult and/or inefficient to couple pump power into the optical fiber and add complexity to the pump power coupling system.
Furthermore, when the optical fiber generates and propagates a high power optical signal and when the optical power exceeds Raman threshold level, the signal light is shifted to longer wavelength via Simulated Raman scattering, inducing power loss at the operating wavelength and preventing further power buildup.
The problem of Simulated Raman scattering is typically solved by increasing the fiber core diameter and lowering its numerical aperture. However, optical fibers with low numerical apertures and large core diameters are very susceptible to the bend-induced losses. In addition, because of non standard dimensions, these fibers are difficult to handle because they are not compatible with existing fibers or fiber handling technologies (fiber cleavers and fusion splices). Finally, when such fiber is fusion spliced to an optical fiber with a smaller core diameter, the signal power does not couple effectively into a smaller core of the second optical fiber, and a large amount of signal power is lost at the splice due to mismatch between the core sizes.
Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with, and connection to, optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). The polarization characteristic (single polarization) propagates one, and only one, of two orthogonally polarized polarizations within a single polarization band while suppressing the other polarization by dramatically increasing its transmission loss.